What is a lightyear?

Basically, a light year is the distant that light travels in one year. It has been calculated at 5.88 trillion miles or 9.46 trillion kilometres.

Why use lightyears?

There is a very logical explanation for the use of light years: On Earth it is fine to use miles or kilometres because the distances between objects are not that great but can you imagine calculating the distance between two galaxies using miles or kilometres? For example, the distance to the next nearest big galaxy, the Andromeda galaxy, is 21 quintillion km (or 21,000,000,000,000,000,000 km). This is a number so large that it becomes hard to write and hard to interpret. So astronomers use other units of distance.

However, in our solar system, Light Years are too large to be used so astronomers tend to use the Astronomical Unit(AU). The AU is calculated as the average distance between the Earth and the Sun. It is approximately 150,000 km (93,000 miles). Mercury can be said to be about 1/3 of an AU from the Sun and Pluto averages about 40 AU from the Sun. The AU, however, is not big enough of a unit when we start talking about distances to objects outside our solar system.

For distances to other parts of the Milky Way Galaxy (or even further), astronomers use units of the light-year or the parsec . The light-year we have already defined. The parsec is equal to 3.3 light-years. Using the light-year, we can say that :

  • The Milky Way Galaxy is about 150,000 light-years across.
  • The Andromeda Galaxy is 2.3 million light-years away.

There is no star that lies precisely one light year away from our solar system. The closest neighbouring star is Proxima Centauri at 4.2 light years distance. Next is double-star Alpha Centauri A and Alpha Centauri B, 4.3 light years away. Knowing how fast light travels, one can begin to appreciate the great distances of the universe when speaking in terms of light years. For example, the light given off by Alpha Centauri today will take 4.2 years to reach earth, so when we observe that star, we are seeing it as it was 4.2 years ago. In essence, we are looking back in time when we look out into space.